1. Field of the Invention
The invention relates, in general, to a stall speed indicator for fixed-wing aircraft and, in particular, to an indicator which provides a continuous readout of the airspeed at which stall of each particular aircraft can be expected to occur.
2. Description of Prior Art
Aircraft stall, a situation that exists when the lift of the aircraft wing is less than the apparent weight of the aircraft, is recognized as a critical aircraft problem. As a result, stall warning and avoidance systems are currently in use in most aircraft systems. Single-engine aircraft frequently employ a reverse-airflow, reed-type aural stall warning horn to alert the pilot when an aircraft stall is imminent. Another conventional warning system involves an annunciator light or horn activated electrically by a micro-switch which is mechanically linked to a small airflow-sensing vane located on the leading edge of an aircraft wing. A third type is used on more sophisticated, high-performance aircraft and includes a wing-mounted lift transducer and angle-of-attack indicator which displays the aircraft angle-of-attack in terms of degrees or percentages of stall angle, or referenced to a desired angle-of-attack.
Some of the difficulties with these prior art devices include the inability to provide continuous stall avoidance information in units of speed, particularly the critical margin below the aircraft's indicated airspeed. In addition, the prior art devices do not directly consider power effect or increase in the potential stalling speed resulting from lift generated by the horizontal tail surfaces. The present invention, which is the subject of the test report USA AEFA Project No. 81-06-1, Final Report, "Preliminary Airworthiness Evaluation of a National Aeronautics and Space Administration Automated Stall Warning System for an OV-1 and the Aircraft", July 1984, incorporates not only the horizontal tail lift and power effect but also normal and longitudinal accelerations, aircraft weight, and wing flap extension into the stall speed computation. Also, most of the prior art indicators require the mounting of a measuring device, such as a stagnation pressure point sensor, on the exterior of the aircraft. The present invention does not require such external appendages on the aircraft.